The present invention relates to a hydrostatic transmission (hereinafter referred to as HST) for vehicle that is interposed in a drive-power transmission path between a drive-power source and a driving axle, and a power train for vehicle between the drive power source and the driving axle.
It is known that the HST interposed in the drive-power transmission path between the drive power source and the driving axle is provided with a PTO unit for driving a working device. FIG. 9(a) is a model view illustrating a drive-power transmission in the arrangement that a conventional HST with a PTO unit is applied to a vehicle that has a front axle serving as a driving axe and is provided on the front side of the, vehicle with a mower or any other working device.
As illustrated in FIG. 9(a), the HST with the PTO unit includes a hydraulic pump unit with a pump shaft operatively connected to the drive power source, a hydraulic motor unit with a motor shaft for outputting the drive power through the motor shaft whose speed is non-stepwisely varied in cooperation with the hydraulic pump unit, a PTO unit with a PTO shaft operatively connected to the pump shaft, and an HST housing accommodating the hydraulic pump unit, the hydraulic motor unit and the PTO unit, in which the PTO shaft has a front end extending forwardly through the HST housing.
In some cases, a demand exists for a wide range of speed change of the driving axle and reduced load applied to the HST serving as a main speed change device. In that case, a mechanical transmission serving as an auxiliary speed change device is additionally interposed between the HST as the main speed change device and the driving axle. FIG. 9(b) is a model view illustrating a drive-power transmission path between the drive power source and the driving axle (front axle) in which the HST with the PTO unit and the mechanical transmission are interposed.
Here, comparing the distance between the front end of the PTO shaft and the front axle (hereinafter referred to distance L) in the arrangement of FIG. 9(a) with the distance L of the arrangement of FIG. 9(b), the former arrangement is: L=L1, and the latter arrangement is: L=L1+L2, in which L2 represents the length of the mechanical transmission with respect to a fore-aft direction of the vehicle.
The front end of the PTO shaft is connected to the mower or any other working device via transmission parts such as a connecting rod with a universal joint. Accordingly, the variation of the distance L necessitates the modification of the transmission parts, the working device and any other associated parts.
Taking for example the vehicle that is provided with the mower as the working device having an elevation function, the variation of the distance L invites not only variation of the length of the connecting rod but also variation of the elevation height of the mower.
That is, since the front end of the PTO shaft serves as a fulcrum for the mower during the upward or downward movement, a simply elongated the elongation of the transmission shaft by L2 simply causes the mower to have a different elevation height. Therefore, in order to equalize the elevational height of the mower between the vehicles of FIGS. 9(a) and 9(b), there arises a necessity to modify a hydraulic piston for elevation of the mower or any other parts.
There thus exist the arrangements with only the main speed change device interposed in the drive-power transmission path, and both the main and auxiliary speed change devices interposed therein. In either arrangement, a demand exists for non-variation of the distance between the front end of the PTO shaft and the driving axle. In other words, a demand exists for the arrangement holding the distance between the front end of the PTO shaft and the driving axle constant regardless of the distance between the driving axle and the main speed change device.
The auxiliary speed change device is an optional member that is provided according to a specification of the vehicle. Therefore, regarding parts constituting the power train between the drive power source and the driving axle excepting the auxiliary speed change device, it is preferable to render those parts commonly usable as many as possible for both arrangements with and without the auxiliary speed change device.
The present invention has been conceived in consideration of the above prior arts. It is an object of the present invention to provide an HST that is capable of effectively limiting the variation in distance between an end of the PTO shaft and the driving axle, even if the distance between the driving axle and the HST is varied.
It is another object of the present invention to provide a power train for vehicle that is capable of being adapted to or matching arrangements with or without the auxiliary speed change device or modifications of the same, or meeting any other demands.